Research that provides a translational understanding of synapse development related to circuit dysfunction will contribute to advances in understanding neurodevelopmental and psychiatric disorders. The proposed studies in this competing renewal application will contribute to this effort by integrating proteomics analytical technologies with cell and circuit functional outcomes to determine mechanisms that underlie typical and atypical synapse development in the forebrain. New data will provide the field with opportunities to discover new molecular targets for intervention. Among disorder-associated genes linked to the synapse is MET, a replicated risk gene for autism spectrum disorder that encodes a receptor tyrosine kinase. MET is integrated within a cell signaling network that is implicated in a number of neurodevelopmental and psychiatric disorders. The proposed studies build upon discoveries made during the current grant period: 1) MET is enriched in developing axons and synapses during the peak of synaptogenesis in rodent and primate forebrain; 2) genetic elimination of Met disrupts dendritic and spine architecture and neocortical interlaminar excitatory drive; 3) the functional promoter variant of MET alters human social-emotional circuit activation, network connectivity and the integrity of certain fiber tracts; and 4) MET interacts directly with a number of synaptic proteins, including ?-catenin. This renewal addresses a major knowledge gap - the mechanisms through which MET contributes to synapse development and circuit vulnerability. The proposed experiments will determine the MET synaptic protein interactome (Aim 1), the influence of disrupted MET signaling more globally on protein expression at the developing synapse (Aim 2), and MET beta-catenin interactions on synapse development (Aims 1 & 2) and functional maturation of neocortical circuits (Aim 3). Specifically, experiments in Aim 1 will determine the MET receptor interactome pre- and postsynaptically during synapse development using co-immunoprecipitation and mass spectrometry. A functional model of MET interactome signaling will be tested during synaptogenesis in vitro. Experiments in Aim 2 will measure the impact of disrupting the pre- and postsynaptic MET interactome on synapse development in vitro. In Met-null mice, protein expression changes at developing and mature neocortical synapses will be determined using global and targeted quantitative mass spectrometry. Aim 3 will measure cell-autonomous influences of MET signaling by using in utero electroporation to manipulate expression of MET and elements of the interactome, beginning with ?-catenin. In vivo functional mapping of input to layer 5 cortico-striatal neurons will be performed. The data generated from the proposed studies will lead to innovative hypothesis-testing regarding fundamental mechanisms of normal and pathophysiological circuit development.